Ornamental onions

Did you know that the Allium genus, including onions, garlic and chives, encompasses about 700 species, many of them wonderfully ornamental?

Each year, it seems, a gardener is fascinated by a different theme – perhaps colour, perhaps the creation of a mood, perhaps a particular group of plants that catches one’s eye.
This year’s fascination came to me inadvertently: I won as a Christmas-party door prize a large basket of assorted bulbs. Though the early-spring daffodils and tulips delighted me, the ornamental onions really captured my attention. As always, my curiosity prompted me to dive into a book: Alliums: The Ornamental Onions, written by Dilys Davies, is thorough and informative.

We all know the Allium genus includes onions, garlic and chives, but did you realize that it encompasses about 700 species and that many are wonderfully ornamental? Easy to recognize by sight and smell, onions bear six-petalled flowers in umbels; rubbing a leaf releases a mild onion scent. Davies cleverly describes an umbel as a floral umbrella, with the flower stalks being the spokes. In the most dramatic species, the umbel is so full of flowers that it forms a huge spherical head. Almost all onions develop an underground bulb to survive summer dormancy – a great way to cope with drought. The leaves of most species are long and ­slender. Overall, ornamental onions thrive in full sun with excellent drainage, with a general fertilizer, such as a rose food.

Let’s take a look at the most popular ornamental onions – from tall to tiny. Allium giganteum, native to central Asia, is indeed the giant of the group, growing 1.5 to two metres. Atop each sturdy stem appears a large, dense sphere of small flowers. Because the stamens are prominent, the flower cluster appears a bit fuzzy in profile. The leaves of A. giganteum wither completely by the time the flowers appear in June, which tells you it likes to be dry when dormant. The best way to ensure this is to grow it in a pot plunged into the garden in spring and lifted and placed in a dry spot for the rest of the season. Left in a coastal garden year-round, this species may rot. (Zones 6 to 10)

One of the most popular flowering onions is the spectacular ‘Purple Sensation.’ (Some taxonomists group it with Allium x hollandicum and some with A. aflatunense, which is in fact a different plant.) It grows to one metre and blooms surrounded by its long, strap-shaped, grey-green leaves. In 1994, we planted 1,000 bulbs of ‘Purple Sensation’ along the Laburnum Walk at VanDusen Botanical Garden. Each year at the end of May, the effect is dazzling when the golden chain trees dangle long cascades of pea-like yellow flowers above the large purple balls of the ornamental onions. Although these large onions are expensive bulbs compared to tulips and daffodils, they persist well – we have hardly added to this planting and it is still spectacular after seven years.

On a smaller scale, Allium ‘Purple Sensation’ is equally handsome in a container. I grow several bulbs in a large pot. The flowers transform into round seed capsules, which are also decorative. I leave the faded stems in the pot, tuck in some bedding plants, and enjoy the added height and form. (Zones 4 to 9) Allium cristophii (also called A. albopilosum) is known as stars of Persia. It opens in June, following behind ‘Purple Sensation,’ but is a shorter plant, reaching 30 to 60 centimetres. First it produces a sphere of a hundred slender buds that create a large pompon. Around the solstice, they gradually pop open one by one, turning into shimmering purple stars. The whole effect is light and airy. By the time the inflorescence is completely open, the leaves are withering – a hint that it also likes very rocky soil or the dry-it-out-in-a-pot treatment. This native of Turkey and central Asia is hardy in zones 5 to 8.

Drumstick allium (A. sphaerocephalon) is easy to grow, throwing up a stout 0.6- to one-metre stalk with egg-shaped flower clusters. It grows over a great range – Europe, North Africa and western Asia – and consequently shows a diversity of flower colour, including greenish, pink and dark red-brown. Those sold in shops are usually purplish. These long-lasting summer flowers are popular with bees and flower arrangers! (Zones 4 to 10) Looking every bit like a fantastic firework, Allium schubertii comes to us from the Middle East. Growing to 30 to 60 centimetres, it forms a large ball of purple flowers in late summer. Because some flower stalks are long and some are short, it has a spidery quality. Davies explains that in the wild, before the seeds have ripened, the main stalk snaps off and sends the inflorescence rolling across the landscape, scattering seed on its way. Experts differ on the cold hardiness of this species. Some say bulbs need protection from frost and some say “hardy to zone 4.” Growing this beauty in a pot and keeping it in a cool, dry place when dormant would guarantee success.

Allium moly (golden garlic or lily leek) should be mid-height, but it flops, so ends up somewhat horizontal – a lax inclination that can be attractive in a container. Its cheerful yellow flowers bridge the gap between the end of spring bulbs and the onset of summer bedding, and it wins a prize for vigour. Because the leaves tend to scorch in hot sun, it is best in a bit of shade. (Zones 3 to 9) Perfect for the rock garden or the front of a sunny bed, Allium oreophilum (is usually called A. ostrowskianum in shops) is a short ornamental onion (to five to 10 centimetres) with loose clusters of carmine red flowers in late spring. Despite its origins in the mountains of central Asia (oreophilum means mountain loving), it is an easy garden plant. The cultivar ‘Zwanenburg’ has brighter flowers. (Zones 4 to 9) That’s just a brief introduction to ornamental onions. Give them a try and I think you’ll be pleased. And don’t worry – they don’t exude oniony aromas unless you rub the leaves. Charming, but never offensive – they make perfect garden guests.